The present disclosure is generally related to welding and plasma spray processes and apparatuses such as may be desirable for fabrication, restoration or repair of metal articles. More particularly, the present disclosure relates to a welding process and an apparatus for producing low oxide bond coatings as well as a method for providing a bond coat during a welding process.
Many types of metals are used in industrial applications. When the application involves demanding operating conditions, specialty metals are often required. As an example, components within gas turbine engines operate in a high-temperature environment. Many of these components are formed from nickel-base and cobalt-base superalloys. Since the components must withstand in-service temperatures in the range of about 1,100° C. to about 1,150° C., the superalloys are often coated with thermal barrier coating (TBC) systems. These coating systems usually include an underlying bond coat applied directly to the superalloy substrate, and a ceramic-based overcoat applied over the bond coat. For a jet engine, the coatings are applied to various superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles.
As with other gas turbine engine parts, gas turbine engine operators find it desirable to repair thermal barrier coated parts periodically to restore them to desirable conditions. Generally, if the part is repairable, it is routed through a repair cycle that includes numerous operations. The repair cycle may include operations such as weld repairs to fill cracks and/or restore tip dimensions, braze repairs to fill cracks and/or restore or change a vane's class, tip repairs to restore abrasive tips, and other steps. The repair cycle also includes reapplication of the bond coat as well as the thermal barrier coating. Accordingly, the repair process generally includes a variety or processing steps requiring different equipment sets. For example, welding may be performed to repair the substrate using various welding equipment and processes specific to welding. The welding may be followed by application of the bond coat, which generally requires transfer of the part to be repaired to a spray cell. As such two separate operations are required for effecting a weld repair and a plasma spray coat application such as the bond coat.
The effectiveness of a TBC system is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate that it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coat, e.g., the microstructure of the bond coat or deficiencies at the bond coat-substrate interface or the bond coat-TBC interface. Thus, carefully controlling the properties of the bond coat is important for adequate and prolonged protection to the substrate.
One problem with current processes is that the bond coatings tend to suffer oxidation during normal thermal spraying in air. The products of oxidation are usually included in the coating and the resulting bond coating is usually harder and more wear resistant. However, the presence of oxides in the coatings detrimentally affect corrosion, strength and machinability. Although various processes such as HVOF have been developed to improve other properties of the coating such as density and porosity among others, these processes are relatively expensive and by their very nature introduce oxygen into its plasma feedstream, thereby providing an inherent mechanism for oxide formation during coating. Moreover, as previously discussed, the use of a separate controlled spray cell is typically required for placement of the part during spray coating.